1. Field of the Invention
The present invention relates to an optical modulator.
2. Related Art Statement
A traveling-wave optical modulator using lithium niobate (LiNbO3), lithium tantalate (LiTaO3) or gallium-arsenide (GaAs) for the optical waveguide has excellent properties and may realize a broadband modulation at a high efficiency. Lithium niobate and lithium tantalate are excellent ferroelectric materials having large electrooptic coefficients and can control light within a short optical path. Factors suppressing the modulation frequency of the traveling-wave optical modulator include velocity mismatch, dispersion, electrode conductor loss and dielectric loss.
The assignee filed a Japanese patent publication 10-133, 159A (U.S. Pat. No. 6,219,469). The publication disclosed a travelling wave optical modulator with an optical waveguide substrate having a thinner portion with a thickness of not more than 10 μm where the optical waveguide is formed. It is thereby possible to realize high-speed modulation without forming a buffer layer made of silicon dioxide, and to advantageously reduce the product “Vπ·L” of a driving voltage Vπ and a length of an electrode “L”.
Further, Japanese Patent number 3088988 disclosed a travelling wave type optical modulator having an electrode for modulation and its terminating portion. The terminating portion has a characteristic impedance Zl lower than the characteristic impedance Zc of the electrode (Zl<Zc).